During engine starting, various approaches may be used to obtain well mixed air and fuel in the cylinder in order to provide more complete combustion and thus less emission of various exhaust constituents. For example, some vehicles may be operated with either or both of liquid and gaseous fuel. In these systems, prior approaches teach that, particularly during engine starting, gaseous fuel may be preferentially used to improve mixing, as compared with liquid fuel, especially during cold (e.g., non-warmed) engine starting conditions. See, for example, U.S. Pat. No. 5,566,653.
However, the inventors herein have recognized some issues with this approach. As one example, when using spark retard to promote increased exhaust temperature and thus faster catalyst light-off, the amount of spark retard may be limited by the combustion characteristics of the gaseous fuel. Namely, when the air-fuel ratio of fresh air to the gaseous fuel is set slightly lean of stoichiometry (to reduce NOx formation), the amount of spark retard may be limited, in part because of the well-mixed state of the gaseous fuel which has substantially no rich pockets that would otherwise aid ignition. In other words, there can be a paradoxical situation using gaseous fuel for engine starting, in that the gaseous fuel's improved mixing helps reduce engine out emissions due to more complete combustion, yet it degrades the engine's ability to increase exhaust heat used to more quickly raise the temperature of an emission catalyst used to react with the engine out emissions.
One example approach to at least partially address the above paradox is a method of controlling an engine having a cylinder with a spark plug, the method comprising: during engine starting, injecting gaseous fuel to the cylinder at least during or before an intake stroke of an engine cycle to form a well-mixed overall lean air-fuel mixture, and then directly injecting liquid fuel to the cylinder at least during one of a compression and expansion stroke of the engine cycle to form a rich air-fuel cloud near the spark plug; and spark initiating combustion of the injected fuels. In one example, a majority of the total fuel burned may be gaseous fuel. In this way, it is possible to achieve both low engine out emissions as the majority of burned fuel is gaseous and well mixed (and lean of stoichiometry), but is it also possible to further retard spark timing and achieve reliable combustion via the rich cloud formed by the liquid fuel injection.
Note that the gaseous fuel injection may be provided in various ways, such as directly or via port or central injection (intake manifold injection), for example. Further, the liquid fuel injection may be provided in various ratios to the fresh air and to the gaseous fuel injection, depending on operating conditions.
Another example approach to at least partially address the above paradox is a method of controlling an engine having a cylinder, the method comprising: for a first combustion event, injecting gaseous fuel without injecting liquid fuel to the cylinder and delivering a first spark retard amount to combust the gaseous fuel; and for a subsequent combustion event, injecting gaseous fuel and injecting liquid fuel to the cylinder and delivering a second spark retard amount to combust the gaseous and liquid fuel, the second spark retard amount greater than the first spark retard amount. In this way, the first event, which may be an initial combustion event from rest where the cylinder did not have combustion on previous cycles, can reliably achieve combustion with low engine out emissions, but then one or more subsequent combustion events, such as after several combustion events with gaseous fuel, can utilize increased spark retard to more rapidly increase catalyst temperature. In particular, the already spinning engine during the subsequent combustion event(s) further improves the ability to achieve reliable combustion with the rich cloud, where the second spark timing is significantly more retarded than would otherwise be possible. Further, in one example, by starting the engine with gaseous fuel, and particularly without over-fueling the first combustion event (e.g., combusting at or lean of stoichiometry), it is still possible to achieve reliable combustion with reduced emissions. Namely, the over-fueling can be reduced due to the avoidance of fuel vaporization issues, such as for the first combustion event from rest.